Electrical machines of this kind which, within the meaning of this disclosure, comprise both electric motors and electrical generators, in particular alternators and starter generators for motor vehicles are generally known. A starter generator is to be understood, in particular, to mean an electrical machine which combines the functions of an electric motor, for example of a starter for a motor vehicle, and an electrical generator, for example an alternator, in a single machine.
Air-cooled electrical machines have, for example in their axial extent, a fan or a fan impeller which is usually directly connected, that is to say connected in a rotationally fixed manner, to the shaft of the electrical machine. Therefore, a cooling air stream is generated by the fan impeller during operation of the electrical machine, said cooling air stream providing a corresponding quantity of cooling air, for the purpose of cooling the electrical machine, as a function of the rotation speed of the machine. The air which is circulated by the fan impeller serves, in particular, to cool the electrical machine at and/or in the housing of said electrical machine.
The drive of the fan impeller requires an additional drive torque. In the case of operation of the electrical machine as an electric motor, this is provided by the motor for example, as a result of which the useable mechanical power which can be output by the motor is reduced. In the case of operation as an electrical generator, the additional fan drive torque can be provided by the drive motor which drives the generator, and therefore cannot be recovered for conversion into electrical energy.
Modern control strategies for electrical machines, in particular for electrical generators such as alternators and starter generators of motor vehicles which are used, for example, as belt-driven auxiliary assemblies in motor vehicles, and modern battery management systems often lead to operation of the electrical machine, which is operated as a generator, at a high rotation speed of the machine, but without demanding electrical power. In such cases, the fan impeller is always concomitantly driven in the case of rigid coupling to the rotor shaft as mentioned in the introductory part. This causes mechanical losses, even though cooling of the electrical machine would not be required.
Various clutch devices can be used, for example, to influence the transmission of torque to an auxiliary assembly of a motor vehicle engine, which auxiliary assembly is driven by a belt drive. Clutch devices of this kind are arranged between a drive disk or an output drive disk and a corresponding shaft in order to thus mechanically uncouple the auxiliary assembly from the drive or output drive disk and therefore from the drive motor.
DE 20 2006 017 623 U1 further discloses an electromagnetic clutch with slip monitoring, the torques of a drive motor being transmitted to an auxiliary assembly in vehicles by said electromagnetic clutch. Slip monitoring is performed by a sensor coil, an electronic monitoring device and a voltage amplifier. The electromagnetic clutch substantially comprises a part which drives it and is connected to the rotor in a rotationally fixed manner, and a stationary magnet coil former into which the magnet coil is inserted. The armature disk, which is separated from the rotor by an air gap, is located opposite the rotor when the magnet coil is not electrically excited. The rotor and the armature disk are separated by a spring element. The armature disk is firmly connected to a part which drives the auxiliary assembly. If the magnet coil is electrically excited, the armature disk is drawn toward the rotor against the force of the spring element, so that a torque can be transmitted. If the magnet coil is not electrically excited, the armature disk becomes detached from the rotor again on account of the restoring force of the spring element, and remains in position.
WO 2010/079386 A1 describes a clutch apparatus and a starter generator which comprises this clutch apparatus. The described clutch apparatus comprises both a one-way clutch and an electromagnetic clutch which are arranged between a belt pulley and an associated shaft. The electromagnetic clutch is controlled by an electromagnetic actuator, which is fixed to a stator, for example a coil, by a current for generating a magnetic field being externally impressed on the coil.
The abovementioned clutch devices share the common feature of having a relatively complex and therefore expensive construction. Furthermore, it is usually necessary to supply additional activation energy, for example in order to supply current to a solenoid which acts as an actuator, for the purpose of shifting these clutch devices, that is to say activating and deactivating these clutches.
Against this background, the object of the present disclosure is to specify an air-cooled electrical machine, in particular an alternator and a starter generator for motor vehicles, which is of particularly simple construction and can be produced in a cost-effective manner while further having a high degree of efficiency and, moreover, being distinguished by being robust and having a long service life.
This object is achieved by an air-cooled electromechanical system having a rotor, comprising a rotor shaft and a field winding, a driver element, and a fan impeller, rotatably mounted on the rotor shaft at a predetermined distance relative to the driver element during a first condition and rotatably mounted on the rotor shaft and coupled to the driver element during a second condition.
It should be noted that the features which are cited individually in the claims can be combined with one another in any desired, technically reasonable manner and identify further refinements of the invention. The description additionally characterizes and specifies the invention, in particular in connection with the figures.
The electrical machine according to the disclosure is particularly suitable for use as an electrical generator in motor vehicles, in particular as a three-phase AC generator, alternator or starter generator.
Further advantageous details and effects of the electrical machine are explained in greater detail below with reference to exemplary embodiments which are illustrated in the figures.